1. Field of the Invention
The present invention relates to instrumentation used for endosteal fixation of a substitute ligament and, more specifically, to a driver for inserting a tapered bioabsorbable interference screw to perform soft tissue attachment to bone such as arthroscopic endosteal fixation of a substitute anterior cruciate ligament.
2. Description of the Related Art
When a ligament becomes detached from a bone, surgery usually is required to reconstruct the ligament. Often, a substitute ligament or graft is secured into bone tunnels to facilitate incorporation and permanent attachment. It is critical that the reattached tissue or the tissue graft be sufficiently secured to withstand the normal loads to which they may be subjected, e.g., those imposed by a patient on the affected joint or region.
Various methods of graft attachment are known, including the use of interference screws to secure the graft against the walls of a tunnel drilled through the tibia and a socket formed in the femur. A strong graft attachment is obtained by using a metal interference screw to wedge a graft bone block to the wall of a graft tunnel formed through the bone, as disclosed in U.S. Pat. No. 5,211,647 to Schmieding. If a bioabsorbable interference screw is used, the graft is often wedged directly against the bone by the screw, without a bone block.
Bioabsorbable interference screws are usually sized so that they are slightly larger than the diameter of the tunnel, so that they dilate the bone tunnel upon insertion. Dilation advantageously compacts the soft cancellous bone between the ends of the tunnel, providing
better fixation. Conventional straight-sided bioabsorbable interference screws have an interference fit of about 1 mm. meaning that about 1 mm of bone is dilated as the screw is inserted into the bone tunnel.
Recent tests have determined that a relationship exists between screw insertion torque and graft fixation strength when inserting interference screws. As disclosed in the study entitled xe2x80x9cCorrelation of Insertion Torque, Load at Failure and Bone Density Utilizing a Soft Tissue Interference Screw with Free Central Quadriceps Tendon Graft in ACL Reconstruction,xe2x80x9d presented at the 1999 International Society of Arthroscopy, Knee Surgery and Orthopedic Sports Medicine Congress by J. P. Fulkerson et al., the correlation of insertion torque to peak load at failure was calculated to be 0.86, and that the fixation strength achieved in each instance in which the insertion torque was greater than 14.5 in/lbs. resulted in a peak load greater than 315 N at failure.
Greater insertion torque can be achieved by increasing the resistance of the interference screw being inserted into the tunnel. Thus, a larger diameter screw relative to the diameter of the bone tunnel provides greater dilation and a greater interference fit, and therefore increased fixation strength. Some larger, conventionally-shaped interference screws, however, have larger tips and are more difficult to align and insert correctly. One example of a bioabsorbable interference screw which provides increased dilation and interference fit without increased difficulty of insertion is disclosed in pending U.S. patent application Ser. No. 09/711,964 filed Nov. 15, 2000 and entitled xe2x80x9cTapered Bioabsorbable Interference Screw for Endosteal Fixation of Ligaments,xe2x80x9d the disclosure of which is herein incorporated by reference.
In light of the recently discovered correlation between insertion force and fixation strength, it would be beneficial if surgeons could determine the amount of force being exerted during insertion of the interference screw or otherwise directly assess the amount of fixation strength provided by the inserted interference screw. If a graft is secured with insufficient fixation strength to withstand expected loads, the patient may experience a limited recovery of the affected joint at best, and may suffer re-injury or separation of the graft from the bone. On the other hand, if too much force is applied to the interference screw during fixation, the bone may become overstressed, resulting in cracks or other damage. Currently, however, no method is available to quantify the torque force exerted during insertion of the interference screw, or to otherwise determine whether or not an acceptable range of fixation strength has been achieved.
The present invention fills the void in the prior art by providing a torque driver which can measure the amount of torque being applied during implanting the screw. The torque driver according to the present invention is similar to a standard driver known in the art having a handle, drive shaft and drive head, a sleeve disposed around the proximal portion of the shaft, and further includes a torque measurement assembly connected between the handle and the sleeve.
The torque measurement assembly includes a proximal cup and a distal cup, and a coil spring encased between the cups, with one end of the spring fixed in the proximal cup and the other end of the spring fixed in the distal cup. The proximal cup is fixed to the distal end of the sleeve, and the proximal portion of the sleeve is fixed in the handle. The distal cup is fixed to the drive shaft, which rotates freely within the sleeve. Thus, the drive shaft and distal cup move together as one unit while the handle, sleeve, and proximal cup move together as one unit. The two units are physically connected to each other only by the torque spring.
When the torque driver is used to insert an interference screw, the driver head is inserted into the socket of the screw and the driver is turned, the rotational force exerted to drive the screw causes the torque spring to tighten. A torque scale is marked on one of the proximal and distal cups in accordance with the resistance of the torque spring, while a reference marking is provided on the other. As the spring tightens, the reference mark moves along the scale to indicate the torque measurement exerted on the spring.
Since the torque exerted to drive an interference screw into bone is related to the resistance met by the screw in entering the bone, using the present invention, a surgeon can be confident that a desired minimum torque load is being applied which translates to a correspondingly desirable level of fixation strength for the graft. The surgeon is made aware if inadequate fixation strength is measured during fixation of the screw, whereupon backup fixation devices are indicated and can be implanted to secure the graft, or alternatively, an interference screw having a larger diameter can be used instead. Also, the surgeon is made aware if too much torque is being exerted, which creates a risk of splitting or otherwise damaging the bone at the fixation site.
Preferably, though not necessarily, the interference screw used with the present invention is a tapered, elongated bioabsorbable interference screw, the taper of the screw extending along substantially the entire length of the elongated threaded screw. The taper of the bioabsorbable interference screw of the present invention advantageously facilitates insertion of the tip of the screw, while providing superior fixation resulting from an enlarged back end. Upon insertion, the bioabsorbable interference screw of the present invention fills all but 5-10 mm. of the tunnel, thereby providing increased fixation strength while also promoting healing.
Additionally, the preferred interference screw used in conjunction with the present invention includes a head provided with a specially designed Delta drive socket for receiving a Delta drive screwdriver or a traditional hex-head screwdriver. The unique driver socket of the interference screw of the present invention optimizes the torque capacity of the screw. To maintain wall thickness, the drive socket may be tapered in correspondence with the tapered outer profile of the device. The taper also permits easy insertion of the tip and shaft of the Delta driver or hex driver (also tapered if screw drive socket is tapered) into the fixation screw.
The tapered bioabsorbable interference screw usable with the present invention is threaded along substantially the entire length of the screw to maximize fixation strength within the tunnel and is preferably, formed of highly crystalline poly-(L-lactic acid) (PLLA) compound. Also, the distal end of the screw, i.e., the end closest to the joint, has a smooth, rounded tip profile so as to minimize abrasion with the graft.
Optionally, the torque driver and interference screw in accordance with the present invention may be both cannulated for insertion over a guide pin. According to this aspect of the invention, the guide pin is inserted through the cannula of the driver and the interference screw, and into the bone tunnel, to thereby guide the interference screw during delivery and installation.
In an exemplary method of ACL reconstruction of the present invention, the graft, such as a hamstring tendon autograft or allograft, is secured by interference screw fixation in a femoral socket formed through the tibial tunnel, as described, for example, in U.S. Pat. No. 5,320,626, the disclosure of which is incorporated herein. The hamstring graft is then drawn taut and secured in the tibial tunnel by insertion of the interference screw using the torque driver of the present invention.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.